PSA (polymer sustained alignment) liquid crystal display devices have a structure in which a polymer structure is formed in a cell in order to control the pretilt angle of liquid crystal molecules. Because of their high-speed responsivity and high contrast, PSA liquid crystal display devices are expected to become next-generation liquid crystal display elements.
To put such PSA liquid crystal display elements to practical use, a problem of “image sticking” caused when a certain image is continuously displayed for a long time needs to be addressed. The cause of image sticking is not simple, and image sticking occurs due to multiple factors. Image sticking is mainly caused by (1) a polymerizable compound left after polymerization, (2) a residual polymerization initiator, and (3) a change in the alignment of liquid crystal molecules (change in pretilt angle).
Such a PSA liquid crystal display element is produced by injecting a polymerizable composition containing a liquid crystal compound and a polymerizable compound between substrates and polymerizing the polymerizable compound while liquid crystal molecules are being aligned by applying a voltage to fix the alignment of the liquid crystal molecules. If the polymerization is not completed and an unpolymerized polymerizable compound is left, image sticking caused by the unpolymerized polymerizable compound occurs. Therefore, the polymerizable compound in the liquid crystal composition needs to be completely polymerized as much as possible to reduce the amount of unpolymerized polymerizable compound.
To facilitate polymerization, it is effective to add a polymerization initiator. However, in existing liquid crystal compositions containing a polymerizable compound, a large amount of polymerization initiator needs to be added in order to reduce the amount of unpolymerized polymerizable compound to the degree that image sticking is prevented. In this case, the voltage-holding ratio of the resultant display element is decreased by the residual polymerization initiator, which results in an adverse effect on the display quality. If a small amount of polymerization initiator is added in order to reduce the amount of residual polymerization initiator and suppress the decrease in a voltage-holding ratio, an unpolymerized polymerizable compound is left because the polymerization does not completely proceed, and thus image sticking due to the unpolymerized polymerizable compound inevitably occurs.
In order to reduce the amount of unpolymerized polymerizable compound by completely curing a polymerizable compound with a small amount of polymerization initiator added, a method of applying a large amount of energy may be employed, the method specifically being performed by applying strong ultraviolet light for a long time in a polymerization process. In this case, however, in addition to an increase in the size of the production apparatus and a decrease in the production efficiency, for example, degradation of the liquid crystal material due to ultraviolet light is caused. Thus, in existing liquid crystal compositions containing a polymerizable compound, it is difficult to reduce both the amount of unpolymerized polymerizable compound and the amount of residual polymerization initiator at the same time. Accordingly, it has been desired to develop a liquid crystal composition containing a polymerizable compound that is completely polymerized without using a polymerization initiator.
It is also known that image sticking is caused by a change in the pretilt angle of liquid crystal molecules in a liquid crystal composition containing a polymerizable compound. Specifically, in the case where a display element is constituted by a flexible polymer, which is obtained by curing a polymerizable compound, when a certain pattern is continuously displayed for a long time, the structure of the polymer changes, resulting in a change in pretilt angle. Since such a change in pretilt angle causes image sticking, it is necessary to use a polymerizable compound capable of forming a polymer that has a rigid polymer structure which does not change.
Hitherto, in order to prevent image sticking by improving the rigidity of a polymer, a display element has been constituted by using a polymerizable compound having a structure such as a 1,4-phenylene group having only a ring structure and polymerizable functional groups (refer to PTL 1) and a display element has been constituted by using a polymerizable compound having a biaryl structure (refer to PTL 2). However, these polymerizable compounds have low compatibility with liquid crystal compounds, and thus deposition of the polymerizable compounds occurs when a liquid crystal composition is prepared. Therefore, it is difficult to apply these polymerizable compounds to a practical liquid crystal composition.
In addition, in order to prevent image sticking by improving the rigidity of a polymer, it has been proposed that a display element be constituted by using a mixed liquid crystal composition containing a bifunctional polymerizable compound and a trifunctional or higher functional polymerizable compound such as dipentaerythritol pentaacrylate or dipentaerythritol hexaacrylate (refer to PTL 3). However, since dipentaerythritol pentaacrylate and dipentaerythritol hexaacrylate have no ring structure in their molecules, the affinity of these polymerizable compounds with liquid crystal compounds is weak and a force that controls alignment is also weak. Accordingly, sufficient alignment stability is not achieved. Furthermore, it is essential to add a polymerization initiator in polymerization of these polymerizable compounds. If a polymerization initiator is not added, these polymerizable compounds are left after the polymerization.
As described above, it is difficult to satisfy characteristics required in liquid crystal compositions containing a polymerizable compound, such as image sticking characteristics of a display element, alignment stability, stability of a composition having such a property that no deposition is produced, and the production efficiency in the fabrication of a PSA liquid crystal display element. This difficulty inhibits the practical use of the display element.